1. Field of the Invention
The present invention relates to the field of electric drives and relates to brushless electric drives and to determining their position.
2. Description of the Background Art
In order to avoid the known disadvantages of electric drives, in particular electric motors with mechanical commutator devices in the form of brushes and the brush arcing that they produce as well as the wear problems of the mechanical commutator device with attendant losses in effective power, the possibilities of power electronics have been exploited to develop so-called brushless electric drives whose susceptibility to failure is significantly lower and whose wear is essentially determined by bearing wear and is therefore very low.
In a brushless electric drive of this kind, the rotor has a number of permanent magnets that move in a rotating magnetic field of the stator, which is provided with phase windings or wire-wound coils. The coils of the stator are activated with power semiconductor switches, in particular power transistors, in particular MOSFETs. The different phase windings of the stator are activated in cyclical fashion. In order to permit a reliable activation and control of a drive of this kind, it is necessary to determine the actual position of the rotor in order to minimize speed differences between the setpoint speed and the actual speed. This also makes it possible to optimally react to changing loads on the drive.
To determine the position of a rotor of this kind in a brushless electric drive, it is possible to use Hall sensors. An embodiment of this kind, however, is cost-intensive and susceptible to failure.
In addition, the voltage induced in the stator winding by the rotary motion of the rotor, the so-called electromotive force (EMF), is used for determining the position and speed. This method, however, is particularly unreliable at low speeds and does not function at the lowest speeds.
It is also conceivable to determine the respective current-induced voltage via the phase windings, which depend in a known way on the change in the supply current and the variable inductance. Measuring methods of this kind, however, can suffer from powerful interference when activated with pulse width modulated signals and also—when special measurement pulses are used—require a particular minimum duration of the measurement pulse in order to be able to determine a specific change in current. Correspondingly long measurement pulses, however, can interfere with the drive and produce torque ripples.